1. Field of the Invention
The present invention relates to a method for gain equalization, and a device and system for use in carrying out the method.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength band of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 .mu.m, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
A gain equalization method is known as measures against the wavelength characteristic of gain of an optical amplifier. This method will be described with reference to FIGS. 1 to 3.
FIG. 1 is a block diagram showing a conventional optical communication system adopting WDM. A plurality of optical signals having different wavelengths are output from a plurality of optical senders (OS) 2 (#1 to #N), respectively, and next wavelength division multiplexed in an optical multiplexer 4 to obtain WDM signal light. The WDM signal light is next output to an optical transmission line 6. The optical transmission line 6 is configured by inserting a plurality of optical amplifiers 8 for compensating for losses and at least one gain equalizer 10 in an optical fiber transmission line 7. Each gain equalizer 10 may be provided by an optical filter. The WDM signal light transmitted by the optical-transmission line 6 is separated into individual optical signals according to wavelengths by an optical demultiplexer 12, and these optical signals are next supplied to a plurality of optical receivers (OR) 14 (#1 to #N), respectively.
Referring to FIG. 2, there is shown an example of the spectrum of the WDM signal light output from the optical multiplexer 4 to the optical transmission line 6 in the system shown in FIG. 1. In FIG. 2, the vertical axis represents optical power, and the horizontal axis represents wavelength. In this example, the optical senders 2 (#1 to #N) output optical signals having wavelengths (.lambda..sub.1 to .lambda..sub.N) respectively. When preemphasis is not considered, the optical powers of the optical signals in all the channels are equal to each other in general. In this example, the band of the WDM signal light is defined by the wavelength range of .lambda..sub.1 to .lambda..sub.N as shown by reference numeral 16.
If each optical amplifier 8 in the system shown in FIG. 1 has a wavelength characteristic of gain in the band 16 of the WDM signal light, a gain tilt or gain deviation is accumulated over the length of the optical transmission line 6, causing an interchannel deviation in signal power or signal-to-noise ratio (optical SNR). In the gain equalization method, the wavelength characteristic of loss of each gain equalizer 10 is set so as to cancel the wavelength characteristic of total gain of the cascaded optical amplifiers 8. This will now be described more specifically with reference to FIG. 3.
In FIG. 3, the broken line shown by reference numeral 18 represents the wavelength characteristic of total gain of the cascaded optical amplifiers 8, and the solid line shown by reference numeral 20 represents the wavelength characteristic of total loss in the gain equalizer(s) 10. In the example shown, the wavelength characteristic of total gain is canceled by the wavelength characteristic of total loss in the band 16 of the WDM signal light, thereby achieving gain equalization in the whole of the optical transmission line 6.
In the case that an EDFA is used as each optical amplifier 8, the wavelength characteristic of gain of the EDFA is asymmetrical with respect to a wavelength axis in general. In contrast, the wavelength characteristic of loss of one optical filter usable as an element of each gain equalizer 10 is symmetrical with respect to a wavelength axis in general. Accordingly, in the case that each gain equalizer 10 includes only one optical filter, the asymmetrical wavelength characteristic of total gain of the cascaded optical amplifiers 8 cannot be compensated. As the optical filter, a dielectric multilayer filter, etalon filter, Mach-Zehnder filter, etc. are known. These filters can be precisely manufactured, and the reliability has been ensured.
As the related prior art to compensate for the asymmetrical wavelength characteristic of an optical amplifier, it has been proposed to configure a gain equalizer by combining two or more optical filters having different wavelength characteristics of loss. With this configuration, the wavelength characteristic of gain can be canceled by the wavelength characteristic of loss with high accuracy in a given band of WDM signal light.
Additional information on the gain equalization method is described in Reference (1) shown below, and additional information on the combination of plural optical filters is described in References (2), (3), and (4) shown below.
(1) N. S. Bergano et al., "Wavelength division multiplexing in long-haul transmission systems", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 6, JUNE 1996, pp1229-1308.
(2) K. Oda et al., "128-channel, 480-km FSK-DD transmission experiment using 0.98 .mu.m pumped erbium doped fibre amplifiers and a tunable gain equaliser", ELECTRONICS LETTERS, 9th June 1994, Vol. 30, No. 12, pp982-983.
(3) T. Naito et al., "85-Gb/s WDM transmission experiment over 7931-km using gain equalization to compensate for asymmetry in EDFA gain characteristics", First Optoelectronics and Communications Conference (OECC '96) Technical Digest, July 1996, PD1-2.
(4) T. Oguma et al., "Optical gain equalizer for optical fiber amplifier", Communications Society Conference, IEICE, 1996, B-1093 (pp578).
The wavelength characteristic of gain of an optical amplifier changes according to operating conditions such as a pumped condition of the optical amplifier and an input power of signal light. In a submarine optical repeater system, for example, there is a case that the input power to an optical amplifier may change because of an increase in optical fiber loss due to aging or because of cable patching for repairing. Such a change in system condition causes a change in operating conditions of the optical amplifier, resulting in a change in its wavelength characteristic of gain. Further, there is a possibility that the wavelength characteristic of gain may deviate from a design value because of variations in quality of optical amplifiers manufactured.
In the conventional gain equalization method using an optical filter having a fixed wavelength characteristic of loss, there arises a problem such that when the wavelength characteristic of gain of an optical amplifier changes from a characteristic shown by reference numeral 18 to a characteristic shown by reference numeral 18' in FIG. 4 because of a change in system condition, the new wavelength characteristic of gain of the optical amplifier does not coincide with the wavelength characteristic of loss of the optical filter, causing an equalization error. The equalization error varies according to a system condition, and a large amount of variations in the equalization error may cause an interchannel deviation in signal power or optical SNR or may remarkably deteriorate a transmission quality in a certain channel.
From this point of view, there has been proposed a method using a variable gain equalizer having a variable wavelength characteristic of loss. As the variable gain equalizer, an optical device using a Mach-Zehnder type band rejection optical filter has been proposed.
However, the conventional variable gain equalizer cannot obtain an arbitrary wavelength characteristic of loss in response to variations in equalization error, so that variations in equalization error due to changes in system condition cannot be sufficiently suppressed.